1) Field of the Invention
The present invention relates to a method of and an apparatus for setting a battery alarm voltage in a battery management server, and more particularly, to a method of and an apparatus for setting a battery alarm voltage for a personal digital assistant operated on a battery that is capable of communicating with the battery management server, which automatically sets the battery alarm voltage based on a residual amount of the battery voltage.
2) Description of the Related Art
In recent years, portable information terminal units such as notebook-size personal computers, personal digital assistants (PDAs), electronic notes, and portable telephones have been rapidly distributed as portably convenient mobile tools. Among them, the PDAs are used as dedicated terminals for the Internet, and are widely used among individuals as mobile tools to manage mails and schedules. Usually, as the PDAs are often carried, the PDAs use a battery as a power source, like the portable notebook-size personal computers.
When the device is continuously used in a state that the battery voltage is low, for example, the power may be lost. In such a case, it becomes impossible to use the device or ends up with losing data during operation of the device. Therefore, a battery alarm voltage is set to give an alarm with a display when the battery voltage drops to a predetermined value. Alternatively, an alarm sound is generated according to a residual amount of the battery.
There are a couple of techniques currently in use to manage the battery voltage. One technique (for example, see Japanese Patent Application Laid-open No. H5-180914) is to accurately keep track of the residual amount of the battery by always monitoring the battery charge. The other technique (for example, see Japanese Patent Publication No. 3311416) is to detect the residual amount of the battery corresponding to an operating condition by forecasting a battery holding time.
However, the conventional devices that manage the battery alarm voltage have the following drawbacks. Conventionally, a user is informed of a shortage of the battery with an alarm or the like, before the battery voltage drops to a complete discharge. When the battery is used for a long time, however, the operating time varies due to an internal degradation of the battery.
FIG. 8A and FIG. 8B are graphs of time-varying discharge curves due to a consumption of the battery. The vertical axis represents the battery voltage (V), and the horizontal axis represents an operating time (sec) of a device using the battery. A battery alarm voltage VA is a voltage that is preset to make an alarm that the battery voltage reaches near to a suspend voltage VS. The suspend voltage VS indicates that the battery voltage within the battery is substantially at a “zero volts” state, that is, a voltage level at which the device is in a non-operating state (i.e., a memory holding state).
When the residual amount of the battery decreases and when the voltage reaches the battery alarm voltage VA, the device generates an alarm. This alarm urges the user to charge the battery. When the battery voltage is consumed and drops to the suspend voltage VS, the device stops operating.
A discharge curve a in FIG. 8A represents the discharge curve of a battery of an unused (i.e., new) state. The battery voltage (i.e., residual amount) drops to the battery alarm voltage VA at t1 seconds. Since then, the voltage drops to the suspend voltage VS at t2 seconds after passing of tx seconds. In other words, the t2 seconds becomes the operating time of the battery when the battery is started in the unused state. As explained above, usually, the operating time of the battery is secured to enable the device to operate for a predetermined period of time tx (about ten minutes), even after the battery voltage drops to the battery alarm voltage VA.
On the other hand, a discharge curve b in FIG. 8B represents the discharge curve of a battery when it is used for a long time. When the battery is degraded, as the slope of the discharge curve becomes sharper, the voltage reaches the battery alarm voltage VA more quickly.
As shown in the example of FIG. 8B, the voltage drops to the battery alarm voltage VA at t1′ seconds, and to the suspend voltage VS at t2′ seconds. In this case, the time t2′ seconds becomes the battery operating time when the battery is degraded. As a result, the device operation time tends to become short (t2′<t2).
When the battery is degraded, the voltage quickly reaches the battery alarm voltage VA that is preset, and therefore, an alarm is generated frequently, despite the fact that considerable operating time (tx′ seconds) still remains before the voltage reaches the suspend voltage VS. Consequently, the charging interval becomes short, which causes an increase in the number of times of charging.